1. Field of the Invention
This invention generally relates to an absorbent that is adapted for the removal of hydrogen sulfide and other sulfur species from liquid and/or gaseous streams and more particularly to a stable iron (II) oxide and/or hydroxide that is particularly adapted to absorb hydrogen sulfide and other sulfur species from liquid and/or gaseous streams. Methods for making and using the absorbent are also disclosed.
2. Description of Related Art
Various liquid and/or gaseous streams, including hydrocarbon streams such as natural gas liquids (“NGL”), crude oil, acid-gas mixtures, carbon dioxide gas and liquid streams, anaerobic gas, landfill gas, geothermal gas, and the like, also often contain significant quantities of sulfur compounds. Some sulfur compounds that are often found in such streams include hydrogen sulfide, mercaptans and dimethyldisulfide. Particularly in the case of hydrocarbon streams, these sulfur compounds generally must be removed in order to meet emission standards and pipeline requirements.
Because of the noxious, toxic and corrosive nature of sulfur-containing compounds, many different products and methods have previously been disclosed for use in removing such compounds from hydrocarbon streams. One such commercially available product is SULFATREAT® brand particulate reactant that is said to be useful for removing hydrogen sulfide and other sulfur contaminants from gases and liquids including, for example, hydrocarbon fuels and geothermal steam for sale to producers of natural gas and the like. SULFATREAT® is a federally registered trademark of M-I L.L.C. of Houston, Tex., and, in stylized form, of Gas Sweetener Associates, Inc. of Chesterfield, Mo. The SULFATREAT® material has a proprietary formulation but is believed to comprise primarily ferric oxide particles having a high surface area. Iron sponge is another commercially available material composed of ferric oxide distributed on wood chips that is being used for sulfur removal in industrial processes.
Another known process for removing hydrogen sulfide from hydrocarbon streams is the use of a caustic scrubber or amine unit. Most of these processes involve the use of an alkaline solution such as sodium hydroxide (NaOH). Compared to these processes, the disclosed stable iron (II) oxide and/or hydroxide system shows greater sulfur capacity when using the same amount and concentration of caustic solutions.
U.S. Pat. Nos. 5,948,269 and 8,404,031 and published application 2001/0,005,981 show the use of iron salts for the synthesis of alkaline iron compounds, such as iron hydroxide, to remove H2S. In these references iron chloride is preferentially used to produce alkaline iron by contact with an alkaline salt (sodium, calcium or magnesium). These alkaline iron compounds are generally solid sorbents and are not stabilized by an alkaline fluid. As a result, they exhibit lower sulfur loading values and are likely oxidized to a ferric compound once removed from solution. For example, U.S. patent application 2001/0,005,981 shows sulfur loading values between 14 and 90% (0.14 to 0.9 times) based on the iron content for a single run. Compared to these processes, the disclosed stable iron (II) oxide and/or hydroxide system is more economical, produces products with fewer impurities (anions) and has the capability to adsorb up to 6 moles of sulfur per mole of iron.
Another commercially available product is disclosed in U.S. Pat. Nos. 7,744,841 and 7,943,105. This absorbent has been found to be particularly effective at absorbing hydrogen sulfide, mercaptans, dimethyldisulfide and other sulfur-containing compounds from various fluids including natural gas, light hydrocarbon streams such as natural gas liquids, crude oil, acid gas mixtures, carbon dioxide gas and liquid streams, anaerobic gas, landfill gas, geothermal and other sulfur-containing streams. This absorbent can be composed of ferrous carbonate, most preferably siderite granules or powdered siderite that is extruded or otherwise aggregated, compacted or formed into pellets, pills or spheres using a minor amount of water and optionally a binder. The ferrous carbonate used to form these particles is generally of a size where 90% pass through a 100 mesh screen, which corresponds to approximately 150 micrometers. The final absorbent preferably has dimensions ranging from about 4 to about 12 mesh or about 1.7-4.7 mm. and is deep red in color. This sorbent is capable of achieving sulfur loading of 10 to 20% by weight of the sorbent (25-50% based on iron content).
Despite the commercial success of the products covered by U.S. Pat. Nos. 7,744,841 and 7,943,105, there is still a need for an improved absorbent that is capable of removing sulfur compounds from liquid and/or gaseous streams, and in particular hydrogen sulfide from hydrocarbon streams.